The hotter an object gets, the more heat it radiates. Simple, right? It's a fundamental physical principle that makes instruments like infrared goggles possible. But now researchers at Harvard have identified a material that defies this principle by actually appearing colder as it warms up.

At around 70˚ Celsius, the compound known as vanadium dioxide (VO2) switches from being an electrical insulator to an electrical conductor. In a study recounted in the latest issue of Physical Review X, researchers led by Mikhail Kats, a graduate student in Harvard's School of Engineering and Applied Sciences, decided to investigate how that transition affects the way VO2 interacts with light and electricity when applied to a wafer of sapphire. Over at ScienceNews, Gabriel Popkin summarizes their findings:

...the researchers heated the vanadium dioxide-sapphire sample and, with an infrared camera, measured how much infrared light the sample emitted as it warmed. The color gradually shifted from blue to red as the sample's temperature increased from 60˚ to 74˚, as is typical for a warming object. But then something strange happened: Even though the sample's temperature continued to rise up to 100˚, the camera readout returned to an icy blue and stayed there."We saw this really dramatic effect," Kats says. "You have an object that at 90˚ looks the same as at 50˚."